1. Technical Field
The present invention relates to a technology for controlling the behavior of various kinds of electro-optic materials such as charged microparticles (hereinafter referred to as electrophoretic particles) by applying them with voltages.
2. Related Art
The electro-optic devices utilizing these kinds of electro-optic materials have been conventionally proposed as display devices for various electronic instruments. For example, JPA-2004-4714 (Paragraph No. 0103 and FIG. 1) and JPA-2003-84314 (Paragraph No. 0003 and FIG. 9) disclose an electro-optic device having an electro-optic layer 50 intervening in a gap between a first substrate 10 and a second substrate 20 facing each other as shown in FIG. 19. The electro-optic layer 50 is a layer in which positively charged black electrophoretic particles and negatively charged white electrophoretic particles are dispersed in a dispersion medium (both omitted from the drawings).
In the surfaces of the first substrate 10 positioned in the reverse side, the surface facing the electro-optic layer 50 is provided with a lot of pixel electrodes 15 (15a and 15b) arranged in a matrix with a distance from each other. Meanwhile, in the surfaces of the second substrate 20 positioned in the observation side, the surface facing the electro-optic layer 50 is provided with an opposed electrode 21 formed in the entire surface. In this configuration, when a positive electric potential VH with reference to the electric potential of the opposed electrode 21 is applied to either of the pixel electrodes 15, the white electrophoretic particles move closer to the first substrate 10 and the black electrophoretic particles move closer to the second substrate 20 resulting in a black grayscale in a portion corresponding to this one of the pixel electrodes 15. As described above, by controlling the dispersion state of the black and white electrophoretic particles for each of the pixel electrodes 15, a desired image can be displayed.
Meanwhile, in this configuration, the electrophoretic particles migrating closer to the second substrate 20 are distributed in an area wider than each of the pixel electrodes 15. For example, as shown in FIG. 19, the case in which the electric potential VH is applied to one pixel electrode 15a and an electric potential VL lower than the electric potential VH is applied to an adjacent pixel electrode 15b (namely, the pixel electrode 15a displays black while the pixel electrode 15b displays white) is assumed. In this case, an electric flux line L1 from the center of the pixel electrode 15a to the opposed electrode 21 extends in a direction substantially perpendicular to the surface of the pixel electrode 15a. However, an electric flux line L2 from the periphery of the pixel electrode 15a is vent towards the pixel electrode 15b in consequence of the electric potential VL applied to the pixel electrode 15b as shown in FIG. 19. As a result, the black electrophoretic particles moving closer to the second substrate 20 are distributed in an area Rb larger than an area R. Therefore, a high-definition display is problematically inhibited. For example, a black line in a white background is displayed wider than a white line in a black background. Note that, although the electro-optic device utilizing the electrophoretic particles is particularly described here, the same problem can arise in electro-optic devices utilizing other electro-optic materials such as liquid crystal.